Composite materials comprising thermosetting or thermoplastic synthetic resins and reinforcing fibers dispersed uniformly therein are superior in physical properties (particularly tensile strength and impact resistance, so are widely used as panel materials, outer plies, container materials, etc.).
In forming any of such composite materials into a desired shape, it is formed beforehand into a sheet-like or agglomerated shape (including a bulky or block-like shape) (hereinafter referred to as "preform"), and the preform is heated before molding. This is a commonly-adopted molding method.
Sheet-like preforms are widely used because they are advantageous in that they can be easily heated uniformly and are superior in press-moldability. However, sheet-like preforms involve the following drawbacks.
1 There has been adopted a method wherein a synthetic resin material is melt-extruded into sheet and impregnation is made while a glass fiber mat or chopped strands are sandwiched in between two such sheets. According to this method, however, there may occur breakage of the reinforcing fibers during the operation, and the manufacturing cost is high. PA0 2 In forming (hereinafter referred to sometimes as "secondary forming") a sheet-like preform into a desired commodity shape, it is necessary to cut the sheet-like preform according to the desired shape, then laminating such cut preforms onto a metal mold according to a desired thickness and press-molding the laminate, so the molding cost is high.
Recently, therefore, the demand for agglomerated preforms have been increasing gradually.
However, it takes considerable time to heat up agglomerated preforms uniformly without a temperature gradient from the exterior surface to the center of the preform, because agglomerated preforms are thick-walled. More particularly, for heating such agglomerated preforms there usually is employed an infrared heater, an electric heater, or hot air, but all of these methods apply heat from the outside of preforms, so a temperature gradient is formed from the outer surface side toward the center in the preforms, and if the heating speed is too high, the outer peripheral portion will assume an overheated state when the temperature of the central portion has reached an appropriate processing temperature.
Consequently, for example in the case of an agglomerated preform using a thermoplastic resin, the outer periphery side may be softened to an excessive degree and fluidized, or in the case of an agglomerated preform using a thermosetting resin, secondary hardening on the outer periphery side will proceed. In both cases, the moldability is deteriorated markedly.
In order to avoid such problems it is required to make the heating rate as low as possible and thereby minimize the foregoing temperature gradient, so a long time is required for raising the temperature up to the molding temperature. As a result, not only the productivity of molded products is deteriorated markedly but also in the case of an agglomerate preform using a thermosetting resin, secondary hardening proceeds and processability deteriorated during such long time.
The present invention has been accomplished in view of such problems, and it is the object thereof to provide a molding method wherein in a heating process performed prior to molding there is included a heating step capable of raising the temperature of an agglomerated preform uniformly and efficiently, and also provide a preform of a fiber-reinforced composite material.